Referring to the general type of filter press mentioned above, if the slurry is initially pumped into the filter press too quickly under too much pressure, this results in the phenomenon known as "blinding" wherein the particulate in the slurry is driven with excessive force into the filter cloth of the filter plates, which disadvantageously clogs the filter cloth. It is therefore desirable to feed the slurry more gradually into the filter press so that the particulate gradually accumulates on the filter cloth to create a particulate bed on the filter cloth. This gradually-accumulated particulate bed filters the remaining slurry upstream thereof.
The aforementioned gradually-accumulated particulate bed is conventionally produced by stepwise increasing the feed pump pressure over time until the slurry pressure eventually reaches a desired end pressure value. However, in order to ensure effective filtering, the magnitude and timing of the stepwise pressure increments are typically determined based on a laboratory analysis of the slurry and the characteristics of the press filter being used. The filter press is typically provided with control devices such as relays, switches and timers which are then configured to produce the desired pressure increments at the desired times.
Thus, the aforementioned stepwise pressure increment approach requires calculation of the magnitude and timing of the pressure increments for each different slurry to be filtered. Moreover, the aforementioned relays, switches and timers introduce undesirable mechanical complexity and require a relatively large amount of space. In addition, the stepwise pressure increment approach requires a preset filtration cycle time (based on the calculated timing parameters) which cannot be adaptively adjusted during the cycle. Further, this approach does not take into account, or adjust for, operational or environmental changes which occur from cycle to cycle.
Hence, although the conventional stepwise pressure increment approach is effective to avoid the "blinding" phenomenon, the benefits of this approach are nevertheless offset to some extent by the disadvantageous factors mentioned above.
One conventional system uses a programmable logic controller (PLC) to control the stepwise pressure increments. This system eliminates some of the difficulties associated with electromechanical control devices, but does not address the other disadvantages mentioned above. A special programming unit is required to program the predetermined cycle parameters into the PLC. Whenever new cycle parameters are required for a new slurry, the programming unit must be temporarily connected to the PLC to program the new cycle parameters into the PLC.
In some conventional systems, a pressure switch is provided in the air supply to the feed pump. Each time the feed pump executes a stepwise pressure increment, the air pressure drops temporarily and causes the pressure switch to activate a timer. However, using this approach, the range of acceptable pressure increments is limited by the sensitivity of the pressure switch. That is, the pressure switch will not trigger the timer unless the pressure increment is sufficient to cause an air pressure drop at least as large as the sensitivity of the pressure switch. Also, the pump pressure is subject to pressure head losses and the like, and consequently the pressure switch can be erroneously actuated.
One known system has attempted to avoid the aforementioned step-wise pressure increment method, and instead has utilized sensors and controls which attempt to regulate the air supply pressure to the pump in response to the slurry pressure leaving the pump so that the pump and slurry pressure are maintained in closer relationship to one another throughout the filter press cycle. This known arrangement, however, is believed to experience operational difficulties caused by the fact that the pump air pressure always attempts to respond to the slurry pressure. In this respect, the Assignee has discovered that in some cycles during filling of the filter press the slurry pressure will suddenly significantly drop prior to again resuming a gradual increase, possibly due to sudden dislodgement of clogs within the press. When these sudden slurry pressure drops occur, it has been observed that if the pump air pressure is allowed to drop due to its responding to the slurry pressure in an attempt to maintain a uniform pressure differential therebetween, then often times can cause a stall condition in the pump and hence seriously disrupt proper operation of the filter press. Further, in this known arrangement, the cycle terminates by activating a timer when a predetermined maximum slurry pressure is reached, which timer permits the press to continue to operate for a preset period of time, such as about two hours, before shutting down the system. This can, in some instances, cause premature shut down of the press prior to complete filling thereof, such as occurs when a premature pressure spike occurs so as to activate the maximum pressure sensor.
Considering the aforementioned approaches to slurry pressure control during the filtration cycle, none of them is capable of adjusting slurry pressure near the end of the cycle, that is, the slurry end pressure. However, Applicants have recognized that adjustment of slurry end pressure facilitates an increase in the filtration cycle efficiency so as to permit shut down of the press only when the press is full.
It is therefore an object of this invention to provide an improved filter press control apparatus, and a method of control, which overcomes many of the advantages associated with prior control methods and apparatus for filter presses.
It is therefore one object of the present invention to provide a slurry press control method and apparatus which can suitably adjust end pressure, and in fact respond to pressure changes after the maximum end pressure has been sensed, to permit continued operation of the filter press and resetting of the end pressure if certain operational conditions are met so as to ensure that the end pressure, when sensed, will permit shut down only when the press is substantially full.
It is another object of the present invention to provide a slurry pressure control method and apparatus, as aforesaid, which is able to sense and respond to, and hence adjust the preset end pressure, such as when the air line pressure decreases, so that the end pressure will still be sufficiently less than the line pressure to permit shut down of the press upon completion of the cycle.
It is a further object of the present invention to provide a slurry pressure control method and apparatus, as aforesaid, which avoids the "blinding" phenomena while also avoiding the disadvantages associated with the prior art approach of stepwise pressure increments.
It is still a further object of the invention to provide a slurry pressure control method and apparatus which is capable of maintaining a substantially uniform pressure differential between the pump and slurry pressures throughout the filling cycle to optimize the filling of the press, with the control method and apparatus being such as to maintain the pump pressure constant when the slurry pressure decreases during the cycle so as to prevent control problems such as pump stall.
It is another object of the present invention to provide a slurry pressure control method and apparatus, as aforesaid, which enables the filter press cycle to automatically proceed substantially unattended between press start up and shut down so that a complete cycle can be carried out without requiring substantial or constant operator control and/or observation.
It is still another object of the invention to provide an improved slurry control method and apparatus, as aforesaid, wherein the press has a visual display and controller provided directly thereon which permits easy inputting of various cycle parameters such as initial start pump pressure, slurry cycle end pressure and the like, and which provides during the operating cycle a visual display of all critical and desirable operating conditions, with the visual display permitting the various conditions to be sequentially visually displayed on the control panel, with such display being automatically or manually scrolled to provide the operator with important information as to the current status of the cycle without requiring continuous monitoring or attendance by the operator.
A method for controlling the filling of a filter press with slurry according to a preferred embodiment of the present invention includes the sequentially executed steps of: gradually increasing the pressure of the slurry supplied to thee press until the slurry pressure reaches an upper pressure limit; recording a trigger point in time at which the slurry pressure reaches the upper pressure limit; and determining whether the slurry pressure subsequently falls below a lower pressure limit within a predetermined period of time commencing at the trigger point. If the slurry pressure does fall below the lower pressure limit within the predetermined period of time, then the above-listed sequence of steps is repeated until the slurry pressure fails to fall below the lower pressure limit within the predetermined period of time, thereby indicating that the press is full and terminating the process.
The aforementioned method is preferably carried out by providing the press of the invention with a control panel thereon which contains suitable control means, such as a suitable microprocessor, capable of responding to a signal which is indicative of the slurry pressure leaving the pump so as to adjust the air pressure supplied to the pump to maintain a substantially constant differential therebetween, except for the air pump pressure being maintained substantially constant when the slurry pressure decreases, with the pump pressure remaining constant until the slurry pressure again builds up so as to permit resumption of the predetermined differential between the pressures. The controller also preferably includes a visual display panel directly on the press for permitting operator-setting of cycle parameters, and for also permitting display of actual cycle parameters and conditions throughout the rather lengthy press filling cycle so as to permit the operator to readily and promptly obtain necessary information without requiring constant attendance or supervision by the operator.
Other objects and purposes of the invention will be apparent to persons familiar with systems of this general type upon reading the following specification and inspecting the accompanying drawings.